1. Field of the Invention
The present invention relates to paperboard packages or cartons suitable for distributing, marketing and heating prepared food products.
2. Description of the Prior Art
To meet complex purity and performance specifications, highly specialized packaging systems have been developed for distributing, marketing and heating food for service and consumption. Many of these packaging systems are based upon a structural substrate folded from a pre-printed and die-cut bleached sulphate paperboard as described by U.S. Pat. No. 4/249,978 to T. R. Baker, entitled "Method Of Forming A Heat Resistant Carton", U.S. Pat. No. 3,788,876 to D. R. Baker et al., entitled "Carton Blanks Printed with a Heat Sealable Composition and Method Thereof" and commonly assigned U.S. Pat. No. 4,930,639 to W. R. Rigby, entitled "Ovenable Food Container with Removable Lid".
To protect the paper package or carton from moisture degradation, due to direct contact with a food substance, the internal surfaces of such a carton are coated with a moisture barrier of one or more continuous films of thermoplastic resin. These films are usually applied to the paperboard web, prior to printing and cutting, as a hot, viscous, extruded curtain. Low density polyethylene (LDPE), polypropylene (PP) and polyethylene terephthalate (PET) are three of the more common thermoplastic resins used for this purpose.
Lids for paperboard-based food cartons may take one of several forms including a top flap that is an integral continuation of the same paperboard sheet or "blank" from which the carton vessel is erected, such a top flap being crease hinged to one sidewall of the carton. Another type of lid is an independent paperboard sheet that is adhesively secured or plastic fuse bonded to the carton vessel sidewalls either by direct attachment to the sidewalls or to a small perimeter flange folded from the upper edge of the side-walls.
However, carton lids of the foregoing description require three separate converting operations following the manufacture of the paperboard: 1) extrusion of the thermoplastic barrier coating; 2) printing of the sales graphics; and 3) die cutting of the carton lid blank. Consolidating these operations into a single operation would offer obvious economic advantages. Moreover, relatively high coat weights are required for an extruded moisture barrier (typically from 11 to 26 pounds per 3000 ft..sup.2 ream) since lighter coat weights usually result in an inconsistent polymer layer thickness or a layer with little or no adhesiveness to the paperboard.
Finally an extruded polymer moisture barrier greatly complicates those recycling procedures necessary to recover the carton fiber constituency.
It is therefore, an object of the present invention to provide a food packaging carton lid which utilizes a specialized non-extruded polymer (water-based/acrylic-based emulsion) to serve the same functions as an extruded polymer but which can be applied in lesser amounts and in the same converting operation or process used to print the sales graphics.
Another object of the present invention is to specify the critical characteristics of a water-based polymer emulsion that may be printing press applied to a food contacting paperboard surface.
Finally, another object of the present invention is to provide a printing press applied polymer coating on paperboard cartons for direct food contact applications that quickly heat seals to itself, to PET, or to an unprimed, clay coated surface.